the limbic system is a set of brain structures that are involved in a variety of neurological and physiological functions such as emotion and memory formation. the outer brain structures included in the limbic system include the parahippocampal cortices, cingulate gyrus, prefrontal cortex, entorhinal cortex, and the insula. the deeper brain structures include the amygdala, septum, the nucleus accumbens of the ventral striatum, and the hippocampus.
a sidebar on neuromodulatory pathways that are related to limbic functioning. the pathway involving norepinephrine begins in the locus ceruleus in the pons, and mediates attentional selection during stress, activating wide areas of the brain. the dopamine pathway starts at the ventral tegmentum in the midbrain and is involved in maintaining working memory and also reward seeking behavior via the mesolimbic system. the serotonin pathway begins in the raphe nucleus in the medulla, and regulates mood and the sleep/wake cycle. finally, the acetylcholine pathway begins in the diagonal band of broca, septum, and nucleus basalis, and is involved in memory formation and cognition.
the hippocampus is a structure in the limbic system that is centrally involved in formation of memories. it has an "archicortex" (a term that denotes a cortical area that has less than 6 layers) that bulges into the lateral ventricle. it receives sensory afferents from various areas of the brain; in particular, the septum and various cortical areas (sensory, premotor, association, cingulate), allowing it to form episodic memories-- the auto-noetic, contextual experiences of an event. this information is then projected to the hypothalamus for neuroendocrine function, as well as back to the cortical areas (prefrontal, cingulate, temporal) for storage of semantic memories: the abstracted, noetic, non-contextual knowledge which is parsed from episodic memories.
the next sections focused on the limbic system's role in mediating sleep / wake states. it introduced the EEG as a tool to measure synchronous brain activity, used as a tool to detect various brain states during different stages of sleep. during the night, humans alternate between stage 1-4 sleep and REM sleep, which have markedly different neurological and physiological features: stage 4 sleep has a high EEG amplitude and low frequency and thus is called slow wave sleep; it also features decreased afferent input to cortical areas, decreased metabolism, and sleepwalking / tossing. on the other hand, REM sleep is characterized by low amplitude, high frequency EEGs, paralysis of large muscles, and an extremely active mind- leading to dreaming and hallucinations.
the states of arousal and wakefulness are mediated by the thalamus, which receive afferents from the cholinergic pontine pathways which are active during waking and REM states, and then project to the cortex. when the pathways that stimulate the thalamus are activated, the thalamus is activated to send information to the cortex. the cortex receives afferents from monoaminergic pathways (norepinephrine, serotonin, dopamine), cholinergic pathways from the basal forebrain nucleus, and the orexin and hypocretin pathways (which are defective in narcolepsy). when these pathways are stimulated, the cortex is activated to process the input from the thalamus.
sleep regulation is mediated by the hypothalamus and SCN. the VLPO nucleus of the hypothalamus inhibits hypothalamic and brain stem nuclei involved in arousal via GABA and galanin, thus inducing drowsiness. the SCN, as covered in the optic system, receives light/dark information from the eyes and projects to the supraventricular zone and dorsal medial nuclei of the hypothalamus, which promotes wakefulness by inhibiting the VLPO nuclei of the hypothalamus via GABA as well as stimulating the orexin pathway via glutamate.
questions
anatomy and neuromodulators...
1. what are the cortical regions included in the limbic system?
2. what are the deeper brain regions included in the limbic system?
3. what are the brain areas involved in planning, cognition, stress, fear, and memory?
4. where does the neuromodulatory pathway for norepinephrine begin and what is it involved in?
5. where does the neuromodulatory pathway for dopamine begin and what is it involved in?
6. where does the neuromodulatory pathway for serotonin begin and what is it involved in?
7. where does the neuromodulatory pathway for acetylcholine begin and what is it involved in?
hippocampus and memory...
8. describe the structure and location of the hippocampus.
9. what are the structures that project afferents to the hippocampus?
10. describe the pathway of neural activity that leads in and out of the hippocampus.
11. where does the hippocampus project efferent neurons to?
12. what are the two types of explicit/declarative memory?
13. where are episodic memories processed and what are they?
14. where are semantic memories processed and what are they?
15. describe the encoding of episodic memories by the hippocampus.
16. describe the formation of semantic memory.
sleep: EEG's and REM...
17. what does an EEG measure?
18. what are the two stages of sleep that humans oscillate between? how often does this occur?
19. what happens to EEG waves when sleep progresses through stage 1 to stage 4?
20. describe the physiological and neurological state during stage 4 non-REM sleep.
21. describe the physiological and neurological state during REM sleep.
22. what types of sleep (REM vs. slow wave) are the first and second halves of sleep dominated by?
arousal pathways, regulation...
23. what pathways are wakefulness and arousal mediated by?
24. what do pathways that activate the thalamus facilitate?
25. what are the specific pathways that activate the thalamus during wakefulness / arousal? when during the day or night are they the most active?
26. what do pathways that activate the cortex facilitate?
27. what are the specific pathways that activate the cortex during arousal / wakefulness?
28. which of these pathways degenerates during narcolepsy?
29. sleep is regulated by which brain structure?
30. describe the specific action of this brain structure in regulating sleep.
31. describe the regulation of sleep via circadian cycles.
answers
1. parahippocampal cortex, cingular cortex, prefrontal cortex, entorhinal cortex, insula.
2. amygdala, septum, ventral striatum / nucleus accumbens, hippocampus.
3. planning: frontal and cingulate cortices. cognition: cerebral cortex. stress: HPA axis, amygdala, hippocampus. fear: amygdala. memory: hippocampus, entorhinal cortex.
4. begins in locus ceruleus in pons, involved in attentional selectivity during stress and activating large areas of brain.
5. begins in the ventral tegmentum in midbrain, involved in facilitating working memory in prefrontal cortex, and reward seeking behavior via the mesolimbic dopamine system.
6. begins in raphe nucleus in medulla, involved in mood regulation and sleep-wake cycles.
7. begins in nucleus basalis, diagonal band of broca, septum, involved in memory formation and cognition.
8. has an archicortex that bulges into the temporal lobe / lateral ventricle.
9. the septum, and cortical regions such as prefrontal, association, sensory, cingulate cortices.
10. parahippocampal cortex, entorhinal cortex, hippocampus, amygdala.
11. from the fornix of the hippocampus to the hypothalamus for neuroendocrine function, and the prefrontal, cingulate, and temporal cortices for memory consolidation.
12. episodic and semantic.
13. processed in the hippocampus and hippocampal cortices; an auto-noetic, context based memory of the experience of an event.
14. non-contextual, noetic, long term representations based on knowledge of the world. encoded in the limbic cortices: anterolateral temporal and ventrolateral prefrontal.
15. sensory cortices project particular experiences, contexts, and relationships to the hippocampus to form the basis for episodic memory formation.
16. episodic memories are projected to the temporal and prefrontal cortices, where they are encoded into semantic memories; patterns and relationships are parsed from episodic memories and stored.
17. synchronous activity of large groupings of neurons which are producing EPSP's or IPSP's in unison.
18. REM sleep and non REM sleep, every 90 minutes.
19. amplitude of EEG waves increases and frequency gets slower.
20. decreased neural afferent input to cortices. decreased metabolism: heart rate, breathing rate, blood pressure. sleepwalking and tossing/turning. inactive mind, relatively active body.
21. decreased EEG amplitude/increased EEG frequency. rapid eye movements. increased metabolism. paralysis of large muscles. hallucinations and dreaming. active mind, inactive body.
22. first half dominated by slow wave sleep and second half dominated by REM sleep.
23. reticular pathways to the thalamus and cortex.
24. transmission of information from the thalamus to the cortex.
25. cholinergic pontine pathways: pedunculopontine and lateral dorsal tegmental nuclei- most active during waking and REM sleep.
26. processing of information from the thalamus.
27. monoaminergic pathways (norepinephrine, dopamine, serotonin pathway), cholinergic pathway from basal forebrain nucleus, orexin/hypocretin pathway from lateral hypothalamus. (base forbes, t rex hypocrit)
28. the orexin/hypocretin pathway.
29. VLPO nucleus of the hypothalamus.
30. inhibits hypothalamic and brain stem nuclei involved in arousal via GABA and galanin; thus inducing drowsiness.
31. the suprachiasmatic nucleus regulates sleep according to light-dark cycles- projecting neurons to supraventricular zone and dorsal medial nuclei of hypothalamus, which promotes wakefulness by inhibiting VLPO nuclei of hypothalamus via GABA, stimulates orexin via glutamate.
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